The present invention relates to a carrier suitable for a development by a magnetic brush.
Development of an electrostatic latent image can be effected by attracting a minus or plus toner frictionally charged on a plus or minus electostatic latent image formed on a photosensitive member respectively. In order to charge the toner a plus or minus charged carrier is used. If the carrier can be optionally varied in its polarity of charge, one kind of neutral toner can be used as a toner chargeable to both plus and minus polarities.
Hitherto, as methods of changing the polarity of charge on a binder type carrier there are disclosed in Japanese Patent Publication (KOKAI) No. 6660/1986 that various kinds of charge controlling agents are incorporated into a binder resin (First method), and in Japanese Patent Publication (KOKAI) No. 100242/1978 and Japanese Patent Publication (KOKAI) No. 79634/1979 that the surface of magnetic powders or the like is coated with a resin containing charge controlling agents (Second method).
In the first method the effect of the charge controlling agent is negligible, because the content of the magnetic powder in the carrier is too much (200 to 900 parts by weight) in comparison with the toner content, so that the magnetic powder strongly influence the polarity of the carrier even if the charge controlling agents are incorporated into the binder resin.
In the second method, as when the binder type carrier is coated with a coating resin containing charge controlling agents or a solution containing them, the carrier is dissolved by the coated resin or a solvent in the solution, the application of the second method to the binder type carrier is substantially impracticable. Therefore, this method is usually applied to inorganic magnetic powder such as iron powder, magnetite powders, ferrite powder and the like or other carriers having a core of material insoluble with solvent, for example glass core. Thus, the second method is not suitable for binder type carriers.